This invention relates to gas flow measurement and, more particularly, to an arrangement for mounting a critical flow device in a gas pipeline for on-site calibration of a gas meter.
The rapidly increasing cost of natural gas since the 1970's has caused profound changes in attitudes towards meter accuracy, especially when gas at high pressure is involved. Since gas pricing has doubled or tripled in real (non-inflated) terms, and has multiplied by 10 to 15 times in inflated currency, there is a significant desire to obtain much more accurate measurement than contemporary meters are able to obtain or maintain. The relative price of natural gas to other fuels, long depressed due to regulatory restraint, is now approaching parity with alternate fuels, assuring even higher interest in accurate measurement. However, all volumetric meters measure actual volume. Thus, meters operating at higher pressures are no more accurate, and are generally significantly less accurate, because calibration is generally performed at or near atmospheric pressure. At higher operating pressures, a given meter can handle an enormous amount of gas, and yet accuracy of the meter is less predictable. To appreciate the cost of such inaccurracy, as an example, if the cost of gas is $5.00 per thousand standard cubic feet, a twelve inch pipeline operating at 900 psig will have an annual error cost of $3,450,000 for only 1% error of the meter. Since meters are seldom better in point accuracy than .+-.1% error at various pressures, even when calibrated under laboratory conditions, and actual field conditions can cause significantly greater error, it is apparent that uncalibrated meters are inadequate in precision of measurement for larger pipelines and higher pressures. It is therefore a primary object of this invention to provide a highly accurate measurement standard coupled with the meter, on site, so that the standard can be used to calibrate the meter under actual conditions.
It is another object of this invention to provide a calibration standard which is capable of establishing the precise flow rate, on site, for various pressures and various rates of flow, as the meter inaccuracy changes as a function of pressure, flow rate, and pipeline conditions.
A critical flow device is a secondary proving standard that has been proven to correlate extremely well with primary testing standards such as the proving bell. Carefully controlled laboratory correlations with huge proving bells have established critical flow devices, also referred to as sonic nozzles, as an excellent, commercially available standard. Sonic nozzles are highly accurate, can be certified on huge proving bells, are permanent (relatively immune to wear and erosion), are compact, portable, and relatively easy to recalibrate. They operate over wide ranges of temperature and pressure on any type of gas mixture. If the sonic nozzle is mounted immediately downstream from the meter to be calibrated, the test pressure need only be fairly constant. It is therefore a further object of this invention to provide an on-site meter calibration arrangement utilizing a sonic nozzle.
In the past, such devices have only been used for laboratory calibration or used as a field calibration standard by temporary insertion into the pipeline. The major reason for temporary insertion is that at any test flow rate other than 100% of demand, it is necessary to bypass (unmetered) the nozzle to fulfill pipeline demand. Further, the sonic nozzle has a pressure absorption ranging from 5% to 20%, depending on the recovery cone design. Both characteristics preclude permanent installation without extra valving as an in-line calibration standard. On the other hand, because meter accuracy varies with pipeline conditions, flow rate, pressure and usage, it would be highly desirable to leave such a sonic device in the pipeline for the purpose of initial calibration and on-site recalibration. It is therefore still another object of this invention to provide an arrangement whereby a sonic nozzle used for meter calibration can be permanently installed in a pipeline.